1. Field of the Invention
The invention relates to magnetic playback apparatus. In particular, the invention relates to tracking control apparatus for aligning a playback head with a record track recorded on a magnetic storage device at an azimuth angle which is different from the azimuth angle of information recorded on adjacent record tracks.
2. Description Relative to the Prior Art
With reference to the magnetic recording art, tracking is the process of keeping a playback head on the path of a track already recorded on a magnetic storage device. The purpose of tracking control is to adjust the position of the playback head relative to the record track or vice versa, so that the playback head is aligned with the recorded track for maximum signal-to-noise output. Although tracking is a requirement of playback apparatus for a variety of magnetic storage devices, tracking control requirements are particularly stringent for slant track tape playback apparatus where a playback head sweeps transversely across a magnetic tape at high speed, as the tape is moving, and for a magnetic disk storage device, where a disk moves at high speed past a playback head.
When playback occurs on apparatus that is different than the apparatus that was used for recording, tracking control has been found to be even more necessary because of mechanical tolerance build-up inherent in different playback apparatus, variations of the scanning plane of the playback head, variations in storage device position, and other secondary tolerances. Tracking control has in the past often been an operator adjustment, particularly whenever playback occurred with apparatus different than the recorder apparatus.
Also known in the art are methods and apparatus for automatic tracking control. When recording signals in parallel tracks, cross talk from track to track can be kept to a minimum for increasing playback signal output if adjacent tracks are separated by a guard band. However, guard bands consume storage space that otherwise could be used for recording.
Even with guard bands, it is still necessary to provide for tracking control. For example, prior art U.S. Pat. Nos. 4,044,388 and 4,120,008 disclose tracking control apparatus for a video tape recorder in which a tracking control head is arranged to overlap adjacent slant tracks separated by a guard band, to sense horizontal synchronization pulses recorded on each track. The synchronization pulses on one track are prearranged with respect to the synchronization pulses on the adjacent track. Pulses on one track follow their corresponding pulses on the adjacent track by a predetermined distance, the length of which depends on whether or not the following pulse is from an odd-numbered or an even-numbered track. Circuitry connected to the tracking control head produces a track-alignment error signal which drives a tape drive capstan motor to control track alignment. The magnitude of the error is related to the respective playback amplitudes of corresponding synchronization pulses on adjacent tracks. The direction of the error is determined by the distance separating corresponding playback pulses.
It is also known in the art to record with azimuth angles of head gaps which are different from track to track, to minimize cross talk. With azimuth recording apparatus, guard bands can be eliminated. In some cases, adjacent tracks may actually overlap to some degree. Because track pitch is smaller with azimuth recording, the aligning of the record tracks with the playback heads is more critical.
U.S. Pat. Nos. 4,184,181 and 4,297,731 disclose azimuth recording of video signals in which playback tracking control apparatus includes a pair of playback heads having gaps positioned at azimuth angles corresponding respectively to alternating signal tracks. Circuitry, coupled to the heads, detects a phase difference between two consecutive horizontal synchronization pulses, each of which is associated with a respective one of two adjacent signal tracks. An error signal, relating to the magnitude and polarity of the phase difference, drives a headwheel motor to control track alignment.
In the aforementioned prior art, tracking control can be achieved automatically only so long as a predetermined signal has been recorded, for example, horizontal synchronization pulses of a video signal. Accordingly, such prior art suffers from a disadvantage in that its tracking control function only has application to the playback of signals in which there is a priori information regarding a component of the recorded signals.